1. Field of the Invention
This invention relates to squarylium dyes useful as antihalation and acutance materials for photographic and photothermographic articles.
2. Background of the Art
Light-sensitive recording materials, such as photographic and photothermographic articles, frequently suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. These light sensitive recording materials typically comprise a photosensitive layer and a substrate such as film base. Image quality degradation occurs when a fraction of the imaging light which strikes the photosensitive layer is not absorbed, but instead passes through to the film base on which the photosensitive layer is coated. A portion of the light reaching the base may be reflected back to strike the photosensitive layer from the underside. This reflected light may, in some cases, contribute significantly to the total exposure of the photosensitive layer. Any particulate matter (including silver halide grains) in the photosensitive element may also cause light passing through the element to be scattered. Scattered light which is reflected from the film base will, on its second passage through the photosensitive layer, cause exposure over an area adjacent to the point of intended exposure. This effect leads to reduced image sharpness and image degradation. Silver-halide based photographic materials (including photothermographic materials) are prone to this form of image degradation since the photosensitive layers contain light-scattering particles (see, T. N. James, The Theory of the Photographic Process, 4th Edition, Chapter 20, MacMillan 1977).
In order to improve the image sharpness of photographic materials, it is customary to incorporate into one or more layers of the material a dye which absorbs light that has been scattered within the coating and would otherwise lead to reduced image sharpness. To be effective, the absorption of this dye must be at about the same wavelength as the sensitivity of the photosensitive layer.
In the case of imaging materials coated on a transparent base, a light-absorbing layer is frequently coated in a separate backing layer or underlayer on the reverse side of the substrate from the photosensitive layer. Such a coating, known as an "antihalation layer," effectively reduces reflection of light which has passed through the photosensitive layer. A similar effect may be achieved by interposing a light-absorbing layer between the photosensitive layer and the substrate. This construction, known in the art as an "antihalation underlayer" is applicable to photosensitive coatings on non-transparent as well as on transparent substrates.
It is also possible to improve image quality by coating a light-absorbing layer above the photosensitive layer of a photographic element. Coatings of this kind, described in U.S. Pat. Nos. 4,312,941; 4,581,323; and 4,581,325; reduce multiple reflections of scattered light between the internal surfaces of a photographic element.
A light-absorbing substance may also be incorporated into the photosensitive layer itself in order to absorb scattered light. Substances used for this purpose are known as "acutance dyes."
Essentially any dye which absorbs light at the wavelength of interest can be used as an antihalation dye, and potentially, as an acutance dye. The restraints on the choice of acutance dyes are greater as the dye must not interfere with the imaging chemistry. Typically this means that the dye cannot cause fogging of the silver in the imaging layer. Some recent patents dealing with antihalation and acutance dyes include U.S. Pat. No. 4,581,325; EP 0,102,781 A2; EP 0,377,961 A1; EP 0,329,491 A2; EP 0,397,435 A1.
Many substances are known which absorb visible and/or ultraviolet light, and many are suitable for image improvement purposes in conventional photographic elements sensitized to wavelengths below 700 nm. However, the use of semiconductor light sources, and particularly laser diodes which emit in the red and near-infrared region of the electromagnetic spectrum, as sources for output of electronically stored image data onto photosensitive film or paper is becoming increasingly widespread. This has led to a need for high quality imaging materials which are sensitive in the near infrared region.
The classes of organic dyes which are commonly employed for antihalation or acutance purposes in ultraviolet and visible light sensitive materials do not readily form stable derivatives and analogues which absorb strongly in the near-infrared. References to visible light herein refer to wavelengths between 400 and 700 nm and references to near-infrared light refer to wavelengths between 700 and 1400 nm, especially 750 to 1300 nm.
Since the human eye is insensitive to near-infrared radiation, coatings of dyes which absorb only at wavelengths longer than 700 nm appear colorless and would therefore be acceptable in imaging materials without any change in absorption during processing. Dyes of this type would be suitable as antihalation and/or acutance dyes in infrared sensitive imaging articles. Heptamethine and longer chain cyanine dyes are known which have absorption maxima in the near-infrared region of the spectrum. However, simple near-infrared absorbing cyanine dyes exhibit an absorption curve which is broadened on the short wavelength side and extends well into the visible region of the spectrum (see for example A. Weissberger and E. C. Taylor Special Topics in Heterocyclic Chemistry, John Wiley and Sons, 1977, page 540). This extended absorption curve into the visible results in an objectionably high blue or green appearance which requires further processing to decolorize or dissolve out the dyes.
U.S. Pat. No. 4,581,325 discloses a group of heptamethine cyanine dyes having a cyclopentene ring in the polymethine chain which are suitable for use as antihalation or acutance dyes in both photographic and photothermographic materials. European Patent Application 377 961 discloses use of polymethine dyes of a particular formula as alternative near infrared antihalation or acutance dyes in either photographic or photothermographic articles.
Unfortunately, these dyes also have tails or some absorption in the visible region of the spectrum and, thus, display a slight purple tint. This tint is undesirable and prevents production of the photographic or photothermographic articles using a clear substrate or film base. Instead a film base with a slight tint is used to counteract the tint of the antihalation or acutance dye.
European Patent Application 569 857 discloses dyes which may be used as infrared or near infrared antihalation dyes in photographic materials. However, once again, the reference teaches that the dyes must be decolored or dissolved out during photographic processing.
It would be desirable to have dyes that absorb strongly in the near infrared yet display little or no coloration in the visible region. These dyes should not interfere or inhibit the effectiveness of the construction during imaging, and, preferably, are easy to prepare.
Some dihydroperimidine squarylium dyes have been known for a number of years. In fact, squaraine dyes containing 2,3-dihydroperimidine terminal groups have been disclosed to absorb light in the near infrared region. K. A. Bello; N. Corns; J. Griffiths, J. Chem. Soc., Chem. Commun., 1993, 452-454. However, this article discusses only the absorbance of the dyes in solvent. In addition, the data shown indicates some residual absorbance of the dye in the visible region. Finally, the article contains no discussion nor suggestion of use of dihydroperimidine squarylium dyes as either antihalation or acutance dyes.